Transparent or translucent inorganic material with high transmission in the 2700-3300 nm wavelength range

ABSTRACT

A transparent or translucent inorganic material, especially a glass-ceramic and/or composite material, is provided with a low average thermal longitudinal expansion coefficient, α, of from -1×10 -6  K -1  to +2×10 -6  K -1  in the temperature range of -50°-700° C., with the following composition (in weight percent): 
     Li 2  O, 2.5-6.0; Na 2  O, 0-4.0; K 2  O, 0-4.0; Na 2  O+K 2  O, 0.2-4.0; MgO, 0-3.0; ZnO, 0-3.0; BaO, 0-3.5; CaO, 0-1.0; SrO, 0-1.0; Al 2  O 3 , 18-28; SiO 2 , 50-70; TiO 2 , 1.0-7.0; ZrO 2 , 0-3.5; TiO 2  +ZrO 2 , 1.0-7.0; and P 2  O 5 , 0-8.0, 
     optionally with coloring components (in weight percent): 
     V 2  O 5 , 0-2.0; Cr 2  O 3 , 0-2.0; MnO, 0-2.0; Fe 2  O 3 , 0-2.0; CoO, 0-2.0; and NiO, 0-2.0, 
     optionally, conventional refining agents, such as As 2  O 3 , Sb 2  O 3 , NaCl, and Ce 2  O 3 , and optionally with high quartz (h-quartz) and/or keatite mixed crystals as the essential crystalline phase. The material exhibits a settable transmission and is suitable, in particular, for the production of plates, pipes, and molded articles, wherein the inorganic material has a water content of less than 0.03 mol/l.

FIELD OF THE INVENTION

The invention relates to a transparent or translucent inorganicmaterial, especially a glass-ceramic and/or a composite material, havinga low thermal expansion coefficient, α, of from -1×10⁻⁶ K⁻¹ to +2×10⁻¹K⁻¹ in the temperature range of -50° C. to 700° C., with the followingcomposition (in weight percent):

Li₂ O, 2.5-6.0; Na₂ O, 0-4.0; K₂ O, 0-4.0; Na₂ O+K₂ O, 0.2-4.0; MgO,0-3.0; ZnO, 0-3.0; BaO, 0-3.5; CaO, 0-1.0; SrO, 0-1.0; Al₂ O₃, 18-28;SiO₂, 50-70; TiO₂, 1.0-7.0; ZrO₂, 0-3.5; TiO₂ +ZrO₂, 1.0-7.0; and P₂ O₅,0-8.0,

optionally with coloring components (in weight percent):

V₂ O₅, 0-2.0; Cr₂ O₃, 0-2.0; MnO₂, 0-2.0; Fe₂ O₃, 0-2.0; CoO, 0-2.0; andNiO, 0-2.0,

If desired, the material may also include conventional refining agents,such as As₂ O₃, Sb₂ O₃, NaCl, and Ce₂ O₃, and/or may also optionallyinclude high quartz (h-quartz) and/or keatite mixed crystals as anessential crystalline phase. The material exhibits a settable wavelengthtransmission and is suitable, for example, for the production of plates,pipes, or molded articles. The invention also relates to a process forthe manufacture of the material and its use.

BACKGROUND OF THE INVENTION

Inorganic materials, such as glass-ceramics, which are transparent ortranslucent in the visible wavelength range and show high stability withregard to temperature fluctuations and which are utilized, for example,as hot plates, are known and commercially available. Theseglass-ceramics are colored by means of coloring oxides, such as MnO₂,Fe₂ O₃, NiO, CoO, Cr₂ O₃, V₂ O₅, and CuO. The effect of these coloringoxides on coloration, i.e., the absorption in the visible wavelengthregion, is discussed in the prior art.

Thus, DE-AS 1,596,858 describes the effect of the individual oxides CoO,Cr₂ O₃, NiO, and Fe₂ O₃, as well as CoO in combination with MnO₂ andCuO, on transmission in the visible wavelength region.

U.S. Pat. No. 3,788,865 examines the effect of combinations of twooxides selected from CoO, NiO, Fe₂ O₃, Cr₂ O₃, MnO₂, and CuO on thetransmission in the wavelength range from 400-700 nm. Moreover, thesimultaneous effect of the three oxides NiO, CoO and Fe₂ O₃, as well asthat of V₂ O₅, on the transmission is described. Although no measuredresults are provided, it is pointed out that the V₂ O₅ -containingglass-ceramic shows good transparency in the IR range.

German Patent 2,429,563 discloses the combined effect of the four oxidesCoO, NiO, Fe₂ O₃, and MnO₂ on the transmission. In the wavelength regionfrom 700-800 nm, a transmission of above 70% is observed, dependent onthe hue of transmitted light, while the IR transmission drops, forcertain wavelengths, to below 10%.

U.S. Pat. No. 4,211,820 describes a brown glass-ceramic wherein the dyeeffect is obtained by TiO₂ and V₂ O₅. Besides these oxides, only Fe₂ O₃is contained therein in minor amounts as the coloring oxide. Thecharacterization of the transmission is inadequate, and one can onlyspeculate that the brown coloring is obtained by measuring thetransmission on 5 mm thick specimens at λ=800 nm. There is no dataregarding transmission in the IR range.

Finally, EP 0 220 333 B1 discloses a transparent glass-ceramiccontaining high quartz mixed crystals which appears black in incidentlight and violet or brown to dark red in transmitted light. It isespecially suited for the production of hot plates, wherein the change,caused by temperature stresses, in the linear thermal expansioncoefficient and transmission is small. The transmission in the IR rangecan be variably adjusted between 800 nm and 2.6 μm, and theglass-ceramic starting material has the following composition (in weightpercent):

SiO₂, 62-68; Al₂ O₃, 19.5-22.5; Li₂ O, 3.0-4.0; Na₂ O, 0-1.0; K₂ O,0-1.0; BaO, 1.5-3.5; CaO, 0-1.0; MgO, 0-0.5; ZnO, 0.5-2.5; TiO₂,1.5-5.0; ZrO₂ , 0-3.0; MnO₂, 0-0.4; Fe₂ O₃, 0-0.2; CoO, 0-0.3; NiO,0-0.3; V₂ O₅, 0-0.8; Cr₂ O₃, 0-0.2; F, 0-0.2; Sb₂ O₃, 0-2.0; As₂ O₃,0-2.0; Σ Na₂ O+K₂ O, 0.5-1.5; Σ BaO+CaO, 1.5-4.0; Σ TiO₂ +ZrO₂, 3.5-5.5;and Σ Sb₂ O₃ +As₂ O₃, 0.5-2.5,

wherein the coloring is made possible by combining oxides selected fromthe group consisting of V₂ O₅, NiO, CoO, MnO₂, Fe₂ O₃, and Cr₂ O₃. Theglass-ceramic is converted from this glass-ceramic starting material bya heat treatment of from 680°-920° C.

As can be seen from the aforementioned patents, the effect of coloringoxides on transmission in the visible range of the h-quartz mixedcrystal-containing glass-ceramics has been examined in detail. Accordingto the art, it is possible, for example, to manufacture in a controlledfashion heatable plates having a thickness of about 4 mm, which appearopaque (black) in incident light and, in transmitted light, violet,brown, up to dark red. Due to these properties, heating elements usedwith a cooking surface or in similar applications are clearly visibleduring operation, while they are not visible through the cooking surfacein the unused condition.

In contrast to the transmission in the visible wavelength region, theeffect of the coloring oxides on transmission in the IR region ofwavelengths higher than 2.6 μm has not been studied in detail.

U.S. Pat. No. 4,057,434 describes an opaque glass-ceramic having athermal expansion coefficient (20°-700° C.) of less than 15×10⁻⁷ /K,with excellent chemical stability, and an infrared transmission at awavelength of 3.5 μm through a polished plate having a thickness of 4.25mm of more than 40%. The glass-ceramic has β-spodumene as the singlecrystalline phase consisting of (in weight percent on oxide basis)2.5-4.5%. Li₂ O; 0.75-3.5%, ZnO; 17.5-21%, Al₂ O₃ ; 65-71%, SiO₂ ; and3.5-6%, TiO₂, and being essentially devoid of alkaline earth oxides andalkali oxides, except for Li₂ O and ZrO₂.

U.S. Pat. No. 4,575,493 relates to an infrared-permeable glass having athermal expansion coefficient of less than 4.24×10⁻⁶ /° C., measured at25°-300° C., consisting of (in mol%) ZnO, 15-30; Al₂ O₃, 2-10; Ta₂ O₅,2-15; and GeO₂, 40-75.

Of the commercially available glass-ceramics, e.g., by Corning, CorningCode 9632, it is known that glass-ceramics colored with V₂ O₅ have avery high transmission in the IR range of 1-2.6 μm, namely, about 80%for specimens with a thickness of 4 mm Also, CERAN® "HIGH TRANS"™,glass-ceramic cooking surfaces sold by Schott colored with V₂ O₅,exhibit a very high IR transmission of about 80% for 3 mm thickspecimens (up to λ=2.6 μm).

In the wavelength range of 2.7-3.3 μm, however, IR transmission drops tovery low values in all glass-ceramics presently on the market, forexample, to below 5% at a wavelength of 2.8 μm for a glass-ceramic platewith a thickness of 3 mm.

For example, when hot plates are used as a cooking surface, IRtransmission is one of the determining variables for good efficiency ofthe cooking system, i.e., short warm-up times and low energyconsumption. The most common heating elements with open heating coilsradiate in the range from 2.7-3.3 μm with 80-95% of their maximumemission. However, precisely this range is absorbed by the materialspresently on the market. Therefore, the radiated energy is not directlyavailable at the bottom of the pan but rather only by way of thermalconduction or secondary radiation of the heated cooking surface.

It is shown in German Patent 2,437,026, as well as "Schott Information2/84", that it is very difficult to optimize a cooking system,especially due to varying configurations of cooking vessel surfaces, butthat such optimization is possible, in substantial part, by changing theIR transmission of the cooking surface.

Also, the development of heating units for glass-ceramic is ongoing. Itis definitely possible, for example, to envision future heating systemsexhibiting a different radiation characteristic from present the heatingunits and/or constructed of several heating zones with differingradiation temperatures and/or characteristics.

These heating systems are to be taken into account, as well, in thedevelopment of an optimized glass-ceramic.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide cooking systems withimproved energy transport from the heating element to the cookware. Theenergy transport is to be improved especially in case of cookware having"poor" properties with regard to heat dissipation by buckled pan bottomsso that the usage properties are not substantially poorer than those in"good" cookware. At the same time, the other essential characteristicsof the cooking surface must be preserved. In the visual range, thetransmission must be set so that turned-on heating elements are visibleeven at low power, but, at full power, a person's eyes are protectedfrom damaging radiation and glare. The plate must absorb light to suchan extent that it appears opaque in the unheated zone in incident light.Also, the material compositions heretofore used with success are to bealtered as little as possible.

In order to obtain the variability of transmission in the IR range, thecombinations of coloring oxides utilized had to be expanded from whatwas previously known; otherwise, it would be impossible tosimultaneously attain strong absorption in the visible range up to about600 nm and variable absorption in the IR range. Of the two coloringoxides,Cr₂ O₃ and V₂ O₅, with strong absorption up to about 600 nm inh-quartz mixed crystal-containing glass-ceramics and with very goodtransparency starting at about 1000 nm, Cr₂ O₃, for example, can beemployed only in very small amounts, since it allows the upperdevitrification temperature to rise greatly, and the associated glassescannot be processed.

However, this is merely one of the many difficulties encountered inattempting to adjust the IR transmission by way of a combination ofcoloring oxides, wherein the adjustability is additionally restricted toa wavelength range of up to 2.6 μm.

It is another object of the present invention to provide inorganicmaterials, for example, glass-ceramics, with an increased totaltransmission, wherein the transmission in the near infrared (600 nm to 2μm) is low due to coloring oxides for coloration in the visible region.

It is a further object of the invention to provide an inorganicmaterial, especially a glass-ceramic which contains h-quartz and/orkeatite mixed crystals, having an adequate stability under extremeconditions with respect to temperature/time stresses as they occur, forexample, when used as a cooking surface, as cookware, or as a dome foran IR detector in airplanes.

A still further object of the invention is to provide a method formanufacturing the inorganic materials discussed above.

The stability with respect to the above-mentioned conversion toglass-ceramics can be specified in various ways. In German Patent2,429,563, a certain ceraming program was repeatedly performed, and thevariation in the linear thermal expansion coefficient was determinedbetween the temperatures 20°-700° C., α_(20/700), depending on thenumber of conversion cycles. By "ceraming" is meant the process by whichmixed crystals are formed in a glass to prepare a glass-ceramic.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

It has surprisingly been found that the objects of the invention areattained by an inorganic material having a variably adjustable andsettable IR transmission in the wavelength range of from about 2.7 to3.3 μm and a water content of less than 0.03 mol/l . The transmission inthe wavelength range of 2.7-3.3 μm for the novel materials is greatlyincreased over known materials. Known materials have been effective onlyin a spectral region of between 1.0-2.5 μm.

According to the invention, it is possible to set the transmission inthe visible and near-infrared spectral region, as indicated above, bythe corresponding selection of nucleating agents, doping withcolor-imparting oxides and a choice of ceraming parameters within largeranges. Also, it is possible to determine and set the transmission inthe spectral region above about 2.5 μm by controlling the water contentof the glass-ceramic. High transmission in this spectral region isadvantageous for many applications of glass-ceramics. Thus, for example,the heat radiation transmission of a conventional heating element, asutilized in cooking areas, may be increased by up to 40% using the novelmaterials of the present invention as a cooking surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the transmission against wavelength for the cooledglass prepared in Example 1 below.

FIG. 2 is a graph of the transmission against wavelength for the glassceramic prepared in Example 1 below.

FIG. 3 is a graph of the transmission against wavelength for the cooledglass prepared in Example 2 below.

FIG. 4 is a graph of the transmission against wavelength for the glassceramic prepared in Example 2 below.

FIG. 5 is a graph of the transmission against wavelength for the cooledglass prepared in Example 3 below.

FIG. 6 is a graph of the transmission against wavelength for the glassceramic prepared in Example 3 below.

FIG. 7 contains graphs of transmission against wavelength for theglass-ceramic prepared in Example 1 below and a state of the artglass-ceramic, (CERAN "HIGHTRANS").

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Conventional glass-ceramic compositions typically exhibit water contentsof0.04 weight % (=0.06 mol/l) or more.

The water content of an inorganic material, such as a glass-ceramic, isdetermined by way of the transmission in the wavelength range between2.5 and 5.0 μm, for example, with an infrared spectrometer, Perkin-ElmerModel 682, on specimens with a thickness of 3 mm, using the followingprocedure (see also FIGS. 1-7):

From the transmission measurement, the T value is determined at thewater band at about 2.8 μm, and the extinction (E) is calculatedaccording tothe formula:

    E=1/d×log.sub.10 1/T.sub.i (cm.sup.-1),

wherein d is the thickness of specimen (cm), and T_(i) is the puretransmission value. Ti is calculated by the equation:

    T.sub.i =T/P,

wherein P is the reflection factor =2n/n² +1, wherein n is therefractive index of the material. The water content (c) is calculatedfromthe equation:

    c=E/ε(mol/l),

wherein the decadic extinction coefficient, ε, in (1×mol⁻¹ ×cm⁻¹) can bederived, for example, from the works of H. Franz, H. Scholze, Glastechn.Berichte [Glass Technology Reports] 36, 1963, p. 350; H. Franz,Glastechn. Berichte 38, 1965, p. 57; and. H. Franz, J. Am. Ceram. Soc.49, 1966, p. 475.

It is known that glass-ceramics based on LiO₂ --Al₂ O₃ --SiO₂ with highquartz mixed crystals as the essential crystalline phase exhibit a lowthermal expansion over wide temperature ranges. To produce thisglass-ceramic, a glass is first melted which contains TiO₂ andoptionally ZrO₂ as the nucleating agents for the subsequentcrystallization, in addition to the primary components LiO₂, Al₂ O₃ andSiO₂ necessary for the high quartz mixed crystal formation. Frequently,GeO₂, MgO, ZnO and P₂ O₅, are also added. GeO₂ improves the glassforming process in asimilar manner to SiO₂. With variations of therelative concentrationsof Li₂ O, MgO, ZnO and P₂ O₅, the thermalexpansion behaviorof the glass ceramic can be controlled. Incorporationof these oxides allows the temperature range for which low thermalexpansion is observed to be broadened or restricted. Addition of thealkalis Na₂ O and K₂ O, as well as BaO, CaO and SrO, improves themeltability of the glass. The glass is then formed directly from themelt, e.g., into plates by rolling or also by drawing, or also intotubes and rods by drawing overcorrespondingly shaped dies. In a secondtemperature process, the so-calledceraming, the high quartz mixedcrystals are formed in the glass and, thus,a thermal expansion is setthat is close to zero. The content of the nucleating agents TiO₂ andZrO₂ in the glass may be adjusted andthe ceraming parameters chosen toobtain a suitable crystalline density such that highly transparentglass-ceramics can be produced with this method. By adding coloringcomponents, such as Fe, Ni, Co, Mn, Cr and V, it is possible to setdesired transmission changes and consequently also adjust the colors ofthe glass-ceramics. To study the glass ceramic properties, for example,specimens were heated at 4° C./min to 720° C., kept at 720° C. for onehour, heated at 2° C./min to 880° C. for 90 minutes, and finally cooleddown to room temperature after switching off the furnace.

Such glass-ceramics find broad applications in areas requiring a highstability to temperature fluctuations and/or dimensional stability attemperature cycles.

In a preferred embodiment, the water content in the glass-ceramic ofthis invention is set to values of less than 0.01 mol/l, particularly toless than 0.005 mol/l, because such low water contents in theglass-ceramic lead to high transmission values in the wavelength rangeof between 2700-3300 nm. For example, transmissions of more than 40%,and preferably more than 60% (for glass articles of 3 mm in thickness),can be obtained.

The following methods are suitable in this connection for the reductionandextensive removal of the OH-ions, i.e., reduction in water content,in glasses which absorb in the wavelength range λ of about 2.5-3.5 μm:

1. Chemical Dehydration

In this process, OH-groups firmly incorporated into the glass network(freeOH-groups and hydrogen bridge bonds) are converted into readilyvolatile compounds. See, for example, U.S. Pat. No. 3,531,205.

Thus, dehydration can be achieved, for example, by adding halogenidesof, e.g., Cl, F, Br, or I, to the blend or by the incorporation ofhalogenidesin the glass. The dehydration occurs in accordance with thefollowing reaction, for example: ##STR1##

The introduction of gaseous halogens is possible but meets withtechnical difficulties.

Another method for chemical dehydration is the addition of carbon, forexample, as pure, elemental carbon, such as graphite; as carbide; or asanorganic carbon compound, such as metal oxalate. Thereby, the OH-groupsare split up with the formation of readily volatile organic compounds,such asmethane, for example. The carbon is preferably added in an amountof 0.01-2.0% by weight of the blend.

However, due to the thus-produced strong reducing conditions, thisprocess,can only be utilized for glass melts substantially devoid ofpolyvalent components and, in particular, also substantially free ofingredients easily reducible to the elemental condition.

2. Physical Dewatering

In this method, the H₂ O partial pressure above the melt is reducedtosuch an extent that the OH-content in the melt is decreased bydiffusion phenomena. See, for example, British Patent No. 948,301.

For this purpose, a vacuum is generated above the melt, for example byevacuation of the furnace space. Preferably, a vacuum of less than 500mbar is applied. However, very expensive apparatus is needed forcontinuous processes using this method inasmuch as the diffusionphenomenatake place substantially only on the surface of the melt, i.e.,convection currents must be produced in the melt in order to attainadequate dewatering.

Another problem of the vacuum method resides in that glass componentshaving a low vapor pressure, such as, for example, alkali oxides, canvaporize, at least in part, and thereby alter the glass composition inan uncontrollable manner.

Therefore, it is more advantageous in any event to pass dry gasesthrough the melt for dewatering purposes. For example, the gases may bebubbled through the melt using suitable means therefor.

For example, by diffusion of the OH-ions into the thus-produced gasbubbleswhich exhibit a very low H₂ O partial pressure, a melt is veryeffectively dewatered at practical expense. In this connection, gasesshould be used which can be removed from the melt easily and withoutreacting with the melt components, such as, for example, He, CO₂, O₂,N₂, NO_(x), and/or noble gases, particularly He and/or O₂.

3. Combination of Chemical and Physical Dewatering

A process, for example, wherein dried gases are introduced into the meltwhich is doped with halogenides is a particularly effective andeconomicalmethod for dewatering the melt to such an extent thatOH-absorption in the infrared spectral region is considerably reduced.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toitsfullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the following examples, all temperatures are setforth uncorrected in degrees Celsius; and, unless otherwise indicated,allparts and percentages are by weight.

The entire disclosures of all applications, patents, and publications,cited herein, and of corresponding German P 43 21 373.1, filed Jun. 26,1993, are hereby incorporated by reference.

EXAMPLES Example 1

A basic glass suitable for producing glass-ceramic, from thecrystallizableglass system of LiO₂ -Al₂ O₃ -SiO₂, for example of thefollowing composition (in weight percent):

SiO₂, 64; Al₂ O₃, 22.1; Li₂ O, 3.5; Na₂ O, 0.6; BaO, 2.0; ZnO, 1.7; MgO,0.5; TiO₂, 2.4; ZrO₂, 1.6; Sb₂ O₃, 1.3; and V₂ O₅ , 0.3,

was made into a melt from conventional raw materials in a 5-literplatinum crucible at 1580° C.

During the melting step, 2.5 1 of CO₂ per minute was introduced into themelt by way of an inlet pipe with several ejection nozzles.

The gas was previously dried by means of silica gel and a molecularsieve column (dew point about -70° C.).

After the blend had been completely melted, 5 l/min of CO₂ was fed intothe melt for another 2 hours.

Subsequently, O₂ was introduced for 30 minutes, likewise at 5 l/min, theoxygen also having been dried beforehand.

Thereafter the melt was refined for 3 hours at 1520°-1600° C.

If covering of the crucible is possible only conditionally, gas purgingof the furnace space and/or of the melt surface is carried out.

In the example, 8 l/min of dried argon (dew point about -70° C.)waspassed over the melt during the entire treatment period of the melt.

After refining, the melt was homogenized, poured into a mold, and cooledat7° C./h.

A part of the cooled glass block was then made into a ceramic, asfollows:

The block was heated at 4° C./min to 720° C., maintained at 720° C. forone hour, heated at 2° C./min to 880° C.,maintained at 880° C. for 90minutes, and finally allowed to cool toroom temperature after turningoff the furnace.

FIG. 1 shows the transmission of the cooled glass; FIG. 2 shows thetransmission of the ceramed (i.e., subjected to ceraming) specimen. Bothspecimens had a thickness of 3 mm.

Example 2

Shards of a halogenide-containing glass of the following composition (inweight percent):

SiO₂, 63; Al₂ O₃, 23.3; Li₂ O, 3.7; Na₂ O, 0.5; MgO, 0.5; ZnO, 1.6; BaO,2.0; TiO₂, 2.4; ZrO₂, 1.7; V₂ O₅, 0.3; and NaCl, 1.0,

were melted at 1580° C. under atmospheric conditions in a platinumcrucible, as in Example 1.

The further procedure corresponds exactly to that of Example 1, exceptthathelium is used in place of CO₂.

FIG. 3 shows the transmission of the cooled glass block obtainedaccording to this mode of operation. FIG. 4 shows the transmission ofthe ceramed specimen (thickness=3 mm).

Example 3

A basic glass, suitable for producing glass-ceramic and having thefollowing composition (in weight percent):

SiO₂, 55; Al₂ O₃, 26.5; Li₂ O, 3.6; K₂ O, 0.6; MgO, 1.1; ZnO, 1.5; TiO₂,2.2; ZrO₂, 1.8; P₂ O₅, 7.0; As₂ O₃, 0.7; and NaF, 1.5,

was "dewatered" in an electrically heated tank furnace at temperaturesof 1580° C. in the melting section, by the introduction of a driedhelium-oxygen mixture (respectively 50 vol %) as well as thesimultaneous passing over of dried air (dew point of the gasesapproximately -70° C).

For this purpose, an agitator for gas feeding with several ejectionnozzleswas introduced in the melting-down section into the melt in sucha way thatthe introduced gas could be distributed uniformly and with anaverage diameter of the bubbles of about 5 mm over a large region of themelting basin. Subsequent refining of the melt took place according toconventionally known methods.

FIG. 5 shows the transmission of the glass, not ceramed, obtainedaccordingto this example. FIG. 6 shows the transmission of the ceramedglass-ceramicspecimen (thickness=3 mm).

FIG. 7 shows the transmission (%) in dependence on the wavelength (nm)of aglass-ceramic according to the state of the art (CERAN "HIGHTRANS")and, incomparison therewith, of a glass-ceramic according to the presentinvention.

FIG. 7 reveals very clearly the significant increase in transmission inthewavelength range between 2500 nm and 3500 nm in a glass-ceramicaccording to Example 1 of this invention, as contrasted withconventional materials.While, in conventional glass-ceramic materials,the transmission in this region can drop to values of below 3% (at about2900 nm), the transmissionin glass-ceramics according to the inventioncan always be maintained at markedly above 60%.

Example of Application of Invention

The use of a material according to the invention for cooking surfaces isone advantageous application of the invention.

At present, a brief heating-up time can be obtained in cooking stoveswith a glass-ceramic top by the use of cookware in close planar contacttherewith. However, if the consumer has cookware with a bulging bottom,the heating-up time is drastically increased. When using a surfaceaccording to the state of the art, an enameled "poor" cookware having acurvature of 4.5 mm requires a heating time of 12.4 minutes to boil 2 lofwater, whereas "good" cookware with a curvature smaller than 1 mmrequires only 10.4 minutes to boil.

With the use of a glass-ceramic according to the present invention, theheating-up time, due to the higher proportion of direct radiation, isshorter for the case of the "good" cookware, requiring only 9.7 minutestoboil 2 l of water. More important is the improvement in case of the"poor" cookware. In this case, a heating-up time of 10.7 minutes isobtained (as compared with 12.4 minutes; see above). The cookingfacility using the materials of the invention thus offers a powerheating ability that is just about independent of the cookware quality.If stainless-steel cookware is utilized, which absorbs heat radiationless well, there is still a considerable improvement over the state ofthe art, i.e., 12.2 heating-up time compared to 13.2 min for 2 l ofwater. The dependency of the heating-up period on the pot quality isreduced to 2/3.

The total transmission for the radiation of grill spits or open heatingcoils (750°-1000° C.), calculated from the spectral emissionof blackbody radiation at a certain temperature and the spectral transmission ofthe cover plate, for a CERAN® color commercial product, can beincreased, for example, from 18% to 25% (and thus by a factor of 1.4),when treated in accordance with the claimed invention.

The preceding examples can be repeated with similar success bysubstitutingthe generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertainthe essential characteristics of this invention and, withoutdeparting fromthe spirit and scope thereof, can make various changes andmodifications ofthe invention to adapt it to various usages andconditions.

We claim:
 1. A transparent or translucent inorganic material with anaverage thermal longitudinal expansion coefficient α of from -1×10⁻⁶ to+2×10⁻⁶ K⁻¹ at a temperature range of from -50° C. to 700° C., with acomposition, in weight percent, of:Li₂ O, 2.5-6.0; Na₂ O, 0-4.0, K₂ O,0-4.0; Na₂ O+K₂ O, 0.2-4.0; MgO, 0-3.0; ZnO, 0-3.0; BaO, 0-3.5; CaO,0-1.0; SrO, 0-1.0; Al₂ O₃, 18-28; SiO₂, 50-70, TiO₂, 1.0-7.0; ZrO₂0-3.5; TiO₂ +ZrO₂ 1.0-7.0; and P₂ O₅, 0-8.0,optionally with coloringcomponents, in weight percent: V₂ O₅, 0-2.0; Cr₂ O₃, 0-2.0; MnO₂, 0-2.0;Fe₂ O₃, 0-2.0, CoO, 0-2.0; and NiO, 0-2.0, optionally with high quartzand/or keatite mixed crystals and, optionally, with conventionalrefining agents, wherein the inorganic material has a water content ofless than 0.03 mol/l and wherein the transmission through a component ofthe material having a thickness of 3 mm is more than 10% in the entirewavelength range between 2700-3300 nm.
 2. The inorganic material ofclaim 1, wherein the water content is less than 0.01 mol/l.
 3. Theinorganic material of claim 1, wherein the water content is less than0.005 mol/l.
 4. The inorganic material of claim 2, wherein thetransmission through a component of the material having a thickness of 3mm is more than 40% in the entire wavelength range between 2700-3300 nm.5. The inorganic material of claim 2, wherein the transmission through acomponent of the material having a thickness of 3 mm is more than 60% inthe entire wavelength range between 2700-3300 nm.
 6. The inorganicmaterial of claim 1, which is a glass-ceramic having high quartz and/orkeatite mixed crystals as an essential crystalline phase.
 7. Theinorganic material of claim 1, wherein the refining agents are As₂ O₃,Sb₂ O₃, NaCl, Ce₂ O₃, or mixtures thereof.